Comments for birthday question

I am not certain how to explain this. No one is obligated to invite you. When you do not make the list of invitees it is a decision the host and/or participants have made. No rule of etiquette or law gives you the right to be invited; only an invitation gives you that right. Your sister has explained the invitation list to you. She is part of "his side of the family" by marriage. Apparently only his side of the family is invited. Based on this description of the invitation list, you should not be invited. I really doubt it is personal at all.